Restricting paraffin formation in producing wells



July 8, 1969 3,454,464

RESTRICTING PARAFFIN FORMATION IN PRODUCING WELLS R.TUGGLE E Filed D96.22, 1967 Tlci.

h\ \\\\L \\N N X w i M 3,454,464 Patented July 8, 1969 United StatesPatent Office 3,454,464 RESTRICTING PARAFFIN FORMATION IN PRODUCINGWELLS Ray Toggle and Richard H. Graves, Houston, Tex., as-

signors to Texaco Inc., New York, N.Y., a corporation of Delaware FiledDec. 22, 1967, Ser. No. 692,865 Int. Cl. E21b 43/00; 010g 43/04 US. Cl.166-304 4 Claims ABSTRACT OF THE DISCLOSURE \Restricting parafiindeposition in flowing oil wells by producing a reservoir liquidcontaining normally liquid and gaseous fractions from which wax tends todeposit,

and effecting a sharp drop in pressure suflicient to refrigerate theflow by vaporization and expansion of normally gaseous fractions andprecipitate the wax as suspended particles which flow away with theproduced oil.

The present invention relates to inhibiting or restricting the depositof massive parafiin wax in producing wells which characteristicallyyield a stream of petroleum from which paraflin wax tends to bedeposited as the production flows to the surface.

As is known, crude petroleum from subsurface reservoirs usuallycomprises a wide range of hydrocarbons typically extending all the wayfrom normally gaseous components such as methane, to the normally solidmaterials such as paraffin waxes. In many producing Wells the presenceof the wax introduces no particular producing problem because, eitherthe relatively low proportion of wax relative to the normally liquidfractions in which it is dissolved, or the nature of the solventconstituents cause it to remain in solution as it passes up through theproducing string.

The present invention is particularly concerned with those crudes whichwhile carrying both gaseous and nor- ,mally solid parafiin in solutiontend-to commence precipitating the paraflin waxes at a temperature whichnor- 'cut away the deposits as particles which are hopefully carriedupwardly by the produced fluids. Alternatively the well may be closeddown and washed with a suitable paraflin solvent. The result in eitherevent is costly in a directly material sense as well as in the loss ofproduction, which results.

In accordance with the present invention we contemplate materially orsubstantially inhibiting or restricting wax formation by effecting asubstantial and sharpchilling or refrigeration of the produced stream ofpetroleum through a substantial range of temperature at a point in theproducing string at which wax deposition therein otherwise tends tocommence. The chilling is effected instantaneously, through a range suchthat wax is precipitated as finely suspended particles which tend toslurry in the oil.

We have found that wax precipitation of this character can be effectedby a controlled pressure drop as for instance by passing the producedstream through a choke appropriate to effect substantially instantaneousexpansion of the contained normally gaseous constitutents of the crudestream. Presumably, therefore, wax precipitation is the result ofrefrigeration by adiabatic expansion of the gases in accordance with theJoule-Thompson effect, as well as by the loss or evolution of the lightgaseous hydrocarbons from the solvent liquid fraction, with absorptionof latent heat of vaporization. There also may be a decrease insolubility of the remaining liquid fractions for normally solid waxes.

In any event, irrespective of the theoretical considerations, by whichwe do not propose to be bound, the produced stream is suddenlycharacterized by a precipitate of solid paraffins which, in the absenceof the choke or restricted orifice would be built up on the wall of thetubing.

We have observed that whereas the solution of wax at or above the cloudpoint tends to deposit upon and adhere to the walls of the tubing, waxaccumulations that have been removed, as for example by scrapping, donot tend to redeposit. The same appears to be true of wax which isprecipitated or crystallized out of solution by autogenous cooling asabove. Moreover, wax does not tend to deposit on surfaces at amaterially greater temperature than the chilled liquid. In any event theprecipitated paraflin wax remains as a slurry or suspension in theproduced oil and is carried to the recovery facilities at the surface.

To the extent that the wax is precipitated it accordingly represents acorresponding decrease in wax deposition on the walls of the productiontubing and a corresponding decrease in the costs attributable to removalof wax deposits in the production tubing.

It is furthermore important to note that the particles of wax thusprecipitated and suspended in the produced stream, provide nuclei ofconsiderable surface for the deposition of solid paraflin insofar asconditions of wax deposition may subsequently be again reached. That isto say, even though conditions of wax deposit are reestablished at someremotely elevated point in the tubing, the overall tendency towarddeposition on the tubing string is greatly reduced, because thedissolved wax has a new and additional surface on which to build in lieuof depositing on the interior surfaces of the tubing. As a result it iscontemplated that the massive wax deposits will be inhibited to anextent greater than indicated by the mere temperature drop effected atthe orifice.

It follows from the foregoing that the orifice or choke is placed at anelevation not lower than the approximate region of incipient waxdeposition in the absence of the orifice, that is to say, at theelevation where the solid or massive parafiin deposit commences to formon said tubing. Location at a point substantially lower than thisobviously fails in some measure to meet the problem of wax deposition.

The flowing petroleum therefore passes through the choke directly into aregion or zone of substantially lower pressure at which a substantialportion of the normally gaseous fraction is spontaneously released fromsolution with concomitant autogenous cooling of the stream andprecipitation of finely divided particles of paraffin wax.

In order to more completely understand the operation of the presentinvention, reference is had to the attached drawing wherein FIG. 1 is amore or less diagrammatic elevation and sectional view of an oil wellplagued by paraflin deposition commencing at an intermediate levelbetween the reservoir and the surface, and FIG. 2 is a correspondingview indicating the condition of the well after the installation of therestrictive orifice.

Referring to FIG. 1 crude petroleum flowing from a subsurface petroleumreservoir reached by the well 10 is carried by way of a production tubeor string 12 to the surface as at 14, where it is distributed tocollecting and treating means not shown.

As the hydrocarbon stream reaches point 16 the gradual cooling which hasoccurred during its flow to the surface has brought the stream to itscloud point. As a result, the production tubing thereabove iscontinuously coated with solid paraffin 18 which progressively builds upinto a flow restricting mass to the obvious detriment of the producingoperation.

Referring now to FIG. 2 the well disclosed in FIG. 1 has beensupplemented by a choke represented by the reference numeral 20. As aresult of appropriate size selection, which may preferably be arrived atby trial and error, flow of the petroleum stream is sharply constrictedso that it, on the lower or upstream side of the orifice 20, asindicated at 22, remains at a pressure and temperature sutficient tomaintain the crude essentially in the liquid phase. On the upper ordownstream side of the choke, however, the product, as indicated at 24,is at a substantially lower pressure determined by the orifice size ofthe choke as well as the flow rate of the well, and the pressures bothin the reservoir and at the surface. As a result the flowing productstream at 24 comprises liquid hydrocarbons interspersed with substantialquantities of released gas 26 which tend to form globules or bubbles ofincreasing size. In addition, as also indicated more or lessdiagrammatically, the autogenous cooling resulting from adiabaticexpansion effects substantial precipitation of the wax as a precipitateor slurry which, as above explained, tends to pass to the surfacewithout depositing on the tubing.

It is to be understood that the wells in question may comprise variousother adjunctive equipment which is of no interest except insofar as itmay indirectly impair the operation of the present invention. Forexample, the provision of pumping equipment in the well or even theconventional use of a choke to control the rate of production or toprotect the well against accident have no significance or effect uponthe present invention insofar as they are conventionally arranged atlocations remote from the present critical location for wax inhibition.

On the other hand, a typical surface choke placed at or near the surfacefor storm protection may adversely limit the benefits available from thepresent invention by restricting the pressure drop otherwise availablethrough the wax choke. That is to say, a surface choke applied to thewell in FIG. 2 might and probably would impose a back-pressure on theproduced stream, which could undesirably decrease the elfect of choke20.

With reference to one installation in accordance with the presentinvention concerning a well producing from a relatively deep reservoir,investigation indicated that the paraffin deposition zone commenced atabout 1875 feet from the surface. That is to say, wax depositionoccurred throughout a zone extending from 1875 feet, to the surface.Further investigation indicated a cloud point approximately 101 F.

A 6 choke was set at a depth of 200feet, the choke being suflicient tosubstantially restrict the flow of produced oil and introduce alocalized pressure drop. Experimental investigation thereafter indicateda reduction or decrease in the paraflin deposition zone of over 400feet. This result is believed phenonemal in view of among other things,the relatively poor location of the choke in proximity to the zone ofwax initiation and the fact that the well in addition employed a surfacechoke which obviously imposed a back pressure which severely limited theavailable pressure drop at the Wax controlling choke.

Various modifications and variations of the invention as hereinbeforeset forth may be made without departing from the spirit and scopethereof and therefore only such limitations should be imposed as areindicated in the appended claims.

We claim:

1. In a producing well comprising means for conducting a stream ofproduced petroleum liquid, containing, in solution, a substantialnormally gaseous fraction and a normally solid paraffin hydrocarbonfraction, from a subsurface petroleum reservoir to surface recoveryfacilities, said petroleum liquid having a cloud point substantiallybelow reservoir temperature and above well head-temperature such thatsolid parafiin deposit normally commences to form on the surface of saidconducting means at an intermediate level in the well as a result ofcooling of said stream as it travels to the surface, the improvementwhich comprises a restrictive orifice installed in said conducting meansat an elevation not substantially below the point where said solidparaffin normally commences to form in the absence of said orifice, saidorifice being selected and located to maintain a pressure and atemperature on the reservoir side thereof suflicient to retain saidpetroleum stream in substantially single phase condition and dischargingsaid stream at a substantially lower pressure on the downstream side atwhich a substantial portion, at least, of the normally gaseous fractionof the petroleum is released from solution with concomitant autogenouscooling of the stream and precipitation of finely divided particles ofwax therein.

2. A producing well as defined in claim 1 wherein the restrictiveorifice is disposed at an elevation just below the point in theproducing stream where the solid paraffin deposit commences to form inthe absence of said orifice.

3. In the production of petroleum liquid containing a normally gaseousfraction and a normally solid paraffin fraction wherein said stream ofpetroleum is conducted from a subsurface reservoir to the surface turnwell tubing with progressive pressure decrease and cooling as a resultof which solid waxy hydrocarbon deposit on the walls of said tubebeginning at an intermediate level therein, the steps which compriseeffecting a sharp drop in the pressure of said stream at a point in saidtube located at an intermediate level therein not substantially belowthe region at which the deposit of normally solid paraffin normallytends to be initiated, conducting said stream to said point of pressurechange at a pressure and a temperature suflicient to retain said streamin a substantially liquid phase condition, discharging said streamtherefrom at a materially lower pressure at which a substantial portionat least, of the normally gaseous fraction is released from solutiontherein with concomitant autogenous cooling and precipitation of finelydivided particles of wax as a precipitate in the stream and conductingsaid stream to the surface without adherence of any substantial portionof said precipitated particles of wax to the well surfaces, such thatwax formation on the Walls of said tube is at least substantiallydiminished.

4. The method according to claim 3 wherein the pressure drop in suchstream is effected just below the point in the producing stream wherethe solid paraffin deposit commences to form in the absence of saidpressure drop.

References Cited UNITED STATES PATENTS 1,992,424 2/ 1935 Halliburton166-45 2,303,823 12/1942 Coberly 166-41 X 2,394,189 2/1942 Kaufman166-41 X 3,269,401 8/ 1966 Scott et a1 137-13 DAVID H. BROWN, PrimaryExaminer.

J. A. CALVERT, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,454,464 July 8, 1969 Ray Tuggle et a1.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 40, "paraffin" should read paraffins line 42, cancel"to". Column 2, line 18, "scrapping" should read scraping Column 3, line56, after "A" insert same line 56, "200" should read 2000 line 61,"phenonemal" should read phenomenal Signed and sealed this 2nd day ofJune 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting ()fficer WILLIAM E. SCHUYLER, JR.

